A reconstituted form of cytochrome P-450 metabolized carbon tetrachloride to phosgene and an electrophilic chlorine species (E-Cl). The reaction was not inhibited by superoxide dismutase, or catalase, and was not supported by cumene hydroperoxide. When the oxygen dependency of the reaction was studied, it was found that the rate of the reaction increased as the oxygen concentration was decreased from 100 to 5%. Decreasing the oxygen concentration below 5% caused a decrease in the rate of the reaction. These results are consistent with a mechanism we call reductive-oxygenation. The first step of the reaction is the cytochrome P-450 dependent reductive-dechlorination of CC14 to trichloromethyl radical. This intermediate is trapped by oxygen to form trichloromethylperoxyl radical which then decomposes to reactive and potentially toxic phosgene and E-Cl. A similar pathway of metabolism may occur with other halogenated compounds.